1. Field of the Invention
This invention relates to a process for producing anhydrous hydrogen fluoride and finely divided silica from waste fluosilicic acid solutions.
2. Brief Description of the Prior Art
In the industrial manufacture of phosphoric acid, phosphate rock is treated with sulfuric acid. This process results in the formation of by-product aqueous fluosilicic acid which is produced in large quantities and is normally discarded. Because of the attendant pollution problems associated with discarding this waste product and the potential value of the waste product as a source of silica and hydrogen fluoride, an economical and efficient process is highly desired for recovery of these important and useful industrial materials from by-product fluosilicic acid. For example, finely divided silica has been historically used as a white pigment in paints for decorating and protecting the surfaces of a wide variety of articles and products such as automobiles, household interiors and household exteriors. Also, hydrogen fluoride is widely used in applications such as polishing, etching and frosting of glass; for pickling copper, brass, stainless and other alloy steels; electro-polishing of metals; manufacture of fluorocarbons; manufacture of cryolite; cleaning stone and brick; purification of filter paper and graphite; dissolving ores and cleaning castings; and the like.
Several electrodialytic procedures have been proposed for recovery of fluorine based acids such as hydrofluoric acid and fluosilicic acid. For example, a process is described in Russian Pat. USSR No. 174,610 (1965) involving an electrodialysis method of concentrating substances, including fluosilicic acid, wherein sulfuric acid is utilized in the anolyte. However, the reference does not indicate whether this procedure is applicable to recovery of anhydrous hydrofluoric acid.
A process for recovery of hydrofluoric acid is described in U.S. At. Energy Comm. ORNL-2308, 14 pp. (1956) in which a by-product aqueous solution of hydrofluoric acid from the hydrofluorination of uranyl dioxide is subjected to electrodialysis followed by distillation to recover hydrofluoric acid. However, this process is not described as useful for recovery of anhydrous HF from an aqueous solution of HF and fluosilicic acid, and the authors believe that the process is not economical for commercial use.
In the process described in U.S. Pat. No. 4,144,158 (Chlanda, Nagasubramanian). Aqueous fluosilicic acid is pyrolyzed to form finely divided silica, a dilute aqueous solution of hydrogen fluoride and fluosilicic acid which is subjected to electrodialysis. Anhydrous hydrogen fluoride is recovered by distillation. While relatively effective as a method for recovering anhydrous hydrogen fluoride, this process suffers from some disadvantages. For example, because of restraints on the concentration of fluosilicic acid, i.e., less than 5%, in the dilute aqueous solution of fluosilicic acid and hydrogen fluoride, the process is restricted to use with pyrolyzation procedures which provide high conversions of fluosilicic acid to hydrogen fluoride and silica. This results in reduced process flexibility and a reduction in the capability of the process to provide differing grades of silica. Furthermore, this disadvantage can also result in high energy expenditures. Another disadvantage of this process is that the concentration of hydrogen fluoride in the electrodialytic output from concentrate cell of the electrodialytic unit must be greater than about 38%. If hydrogen fluoride concentrations are lower than this threshold amount, anhydrous hydrogen fluoride cannot be recovered by distillation and corrosive dehydrating material such as concentrated sulfuric acid must employed.